This invention relates to a method for use in a receiver of a two-dimensional linear modulation data communication system in measuring signal quality of a signal received through a transmission channel of the system.
In a two-dimensional linear modulation data communication system, such as a quadrature phase amplitude modulation (QAM) data communication system, a transmitter of the system two-dimensionally linearly modulates a carrier by a sequence of transmission symbols or codes to produce a modulated carrier signal. The transmitter transmits the modulated carrier signal to a receiver through a transmission channel or path. The transmission channel may be a telephone channel. The receiver receives the modulated carrier signal as a received signal and carries out coherent detection thereon to produce a phase controlled baseband signal and then a reproduced data signal which is reproduced from the phase controlled baseband signal to give a reproduction of the data symbol sequence.
In such a data communication system, it is important upon occurrence of a problem to rapidly discover the causes of the problem and take an appropriate countermeasure to get rid of the causes. Main causes are defects in the transmitter and/or the receiver and degradation of performance of the transmitter channel.
In general, a conventional modem of the transmitter or the receiver produces an interface signal called signal quality. The interface signal has been used in carrying out the in-service monitoring as called in the art to always monitor performance of the communication system during service thereof. The in-service monitoring makes it possible to detect a possibility of a problem and thereby to take a suitable countermeasure.
The interface signal is, however, representative of an overall result of various factors. More particularly, the overall result is a mixture of factors dependent on the state of operation and the performance of the receiver and other factors which result from degradation of the transmission channel and can not be dealt with even by a receiver of the highest possible performance. If the trouble is liable to occur due to disorder of the receiver, the countermeasure should be either to check the operation of the receiver or to switch the receiver to a standby receiver. If the trouble comes from the transmission channel, the countermeasure should be either to use another transmission channel or to reduce the rate of transmission of the data symbols. It is therefore desirable to analyze the causes of trouble and divide them into those resulting from noise superimposed on the modulated carrier signal during propagation through the transmission channel, those resulting from carrier phase jitter introduced to the modulated carrier signal in the transmission channel, and those resulting from carrier frequency offset introduced to the modulated carrier signal in the transmission channel. There has, however, been no method of individually measuring a quality measure of the received signal with the quality measur divided into portions dependent on the noise and on carrier phase disturbance which comprises the carrier phase jitter and the carrier frequency offset.